1. Field of the Invention
The present invention relates to a lithium niobate crystal wafer which can be utilized as a substrate of an optical waveguide or elastic surface wave device, a process for the preparation of the same, and a method for the evaluation thereof. Particularly, it relates to a lithium niobate crystal wafer exhibiting remarkably reduced residual strain, a process for preparing the wafer at high productivity, and a method for evaluating the same.
2. Description of the Prior Art
Up to this time, ferroelectric crystals such as lithium niobate and lithium tantalate crystals have been used for the production of elastic surface wave, Pockels, non-linear and pyroelectric elements and so on, by virtue of their excellent acoustic, electro-optical and pyroelectric effects. Generally, these crystals are grown by the Czocharalski method, annealed for the prevention of cracking in processing, poled for single polarization, and thereafter processed into a cylinder. This cylinder generally has a diameter of 50 to 130 mm and a length of 30 to 100 mm. The cylindrical crystal boule thus prepared is cut into blocks having a specific crystal orientation, or sliced into wafers, followed by lapping and polishing. Then, the resulting products are provided as crystals for various devices.
When lithium niobate crystal is used in the form of a wafer, the wafer generally has a diameter of 50 to 125 mm and a thickness of 0.3 to 5 mm and is used in the production of optical waveguide, surface acoustic wave and pyroelectric elements and so on.
However, residual strain is observed in some of the conventional ferroelectric crystal wafers and the residual strain in such wafers causes unevenness of characteristics of optical devices due to uncontrol of the release of the residual strain in the manufacturing process of the optical devices. In addition, the strain present in an elastic surface wave device brings interference against elastic surface waves and results in unstable temperature dependency. Consequently, the residual strain impedes providing a more stable device for practical use.
In order to overcome these disadvantages, a lithium tantalate crystal wafer for an elastic surface wave device has been reported in Japanese Patent Publication No. 25420/1992, which is reduced in the variation in index of double refraction in the wafer. However, this patent document does not disclose any clear description of the preparation of such a crystal wafer and the object providing the crystal wafer free from the above disadvantages at high productivity has not been accomplished at all.
Further, it has been a common practice to reduce the residual strain of a crystal wafer by annealing a crystal boule just after the crystal growth, but this method cannot serve as a means for reducing the residual strain without fall.
As described above, there has not been found any effective means for reducing the residual strain which brings about unexpected lowering in the quality.